Biscuit baking ovens
Author: Iain Davidson director of Baker PacificOVEN
DESIGNSBiscuit baking ovens, generally known as tunnel ovens, have
long conveyors which carry the dough pieces through a heated box
section baking chamber. Oven lengths vary typically between 25m and
100m long. The conveyor band material is a wire-mesh or a carbon
steel sheet, which turns around large cylindrical drums at each end
of the oven. The conveyor is driven by a variable speed drive at
the oven end which allows the operator to adjust the baking
time.Heating methodsThe baking chamber may be heated directly with
gas burners or electric heaters or by an indirect system using heat
exchangers. Direct heating systems use gas or electric energy,
indirect systems may also use diesel oil fuel as the products of
combustion do not enter the baking chamber.The temperature and
humidity control is divided into zones along the length of the
oven, usually each control zone is between 8m and 20m long. This
enables the temperature and humidity to be set and controlled
throughout the baking process to optimise the conditions for the
establishment of the biscuit structure, moisture content and colour
as the dough pieces travel through the oven.The control of the
humidity in the baking chamber and the removal of moisture from the
dough pieces is accomplished by an extraction system in each zone.
This consists of ducts which draw air and moisture from the baking
chamber through a fan and expel the air through vertical flues
(chimneys) to atmosphere.In some ovens this wet air removed from
the baking chamber can be diverted either to the flue or back into
the baking chamber. This provides moving air within the baking
chamber which can aid heat transfer and contribute to even baking
conditions across the width of the oven. These systems are called
turbulence systems and are mainly used on ovens which have
relatively still air in the baking chamber, for example indirect
radiant ovens and direct gas fired ovens.Ovens are designed to
optimise the heat transfer to the dough pieces in different ways.
We can group the basic designs into the following:1. Radiant
heatingdirect gas fired ovens, electric ovens and indirect radiant
(cyclotherm) ovens2. Conduction heat transferovens with pre-heated
heavy mesh bands and steel bands3. Convection bakingdirect and
indirect convection ovens transfer the heat by blowing hot air jets
onto the products. Also Re-circ ovens which combine convection and
radiant heat transfer.1. Radiant heatingDirect Gas Fired
OvensDirect gas fired ovens are very widely used throughout the
biscuit baking industry. They offer versatility to bake all types
of biscuits, cookies and crackers.
Baker Pacific Direct gas Fired ovenThe direct gas fired oven has
a simple baking chamber of box section with the oven band supported
through the middle of the chamber. Above and below the band are
ribbon gas burners. A gas/air mixture is supplied to the burner
tubes and this is ignited by a spark electrode and burns on a strip
or ribbon across the width of the oven conveyor bandThe heat
transfer in a direct gas fired oven is primarily by radiation from
the gas flames and from the oven top, base and walls of the baking
chamber.
Direct gas fired baking chamberMost types of gas may be used
including natural gas, town gas (manufactured from coal) and LPG
(liquid petroleum gas).The burners operate on a zero gas pressure
system and the pressure of the air supplied to the burners is
controlled by motorised valves or variable speed blowers to
increase or decrease the flame intensity and heat input. Various
designs of corrugated stainless steel strips (or ribbons) are used
to give a range of heat ratings. Woven wire-mesh (metal fibre)
strips are also used for very high infra-red heat ratings.This type
of oven can achieve high heat inputs per square metre of band
surface (up to 40,000 kcal/m2 may be used for cracker baking) and
can successfully utilise any type of baking band, heavy mesh, open
wire-mesh bands and steel bands. Direct gas fired burners are used
to supply band pre-heat where this is required for cracker
baking.
1. Direct gas fired Multi Fibre burners, 2. Burner head with
gas/air mixer, solenoid valve,ignition and flame detection
electrodeThe heat mass and stable radiant heat can be increased by
installing heavy tiles at the top and bottom of the baking chamber.
This arrangement is successfully used in the baking of water
biscuits and crackers requiring very high heat inputs and stable
radiant heating.Summary
1.Direct gas fired ovens provide radiant heat over a wide range
of heat inputs.2.Rapid and responsive control of heat input and
accurate temperature control3.Suitable for all types of products:
crackers, cookies, all types of biscuitsElectric ovensElectric
ovens are constructed in a similar way to direct gas fired ovens,
but use electric heating elements in place of the gas burners.
These ovens have been widely used in the baking industry in some
countries where industry had adequate electricity supply, but
lacked gas, for example China. However most countries, including
China, now use gas predominantly in the baking industry, which is
invariably substantially cheaper than electricity.Electrical
heating has also been used in the first zones of ovens which
required high heat inputs at the start of the baking process and
where diesel oil with an indirect heating system was the preferred
fuel for the main part of the oven.Electric oven baking
chamberSummary1.Electrical energy is clean, does not require
storage and is easily controlled.2.Electric ovens can simply
controlled by thyristor units which vary the current in the
elements and hence the heat input to the baking chamber.3.Suitable
for all types of crackers, cookies and biscuits4.Very expensive to
operate in most markets and not widely used now.Indirect radiant
ovensThe indirect radiant ovens (also known as cyclotherm ovens)
are constructed in separate zones. Each zone is typically 10 20m
long and it has a single burner, heat exchanger and circulation
system for the hot gases from the burner.Each zone has a burner
firing into a burner tube. The hot burnt gases are drawn from the
burner tube through ducts to rows of steel tubes, or ducts, at the
top and bottom of the baking chamber. These radiant tubes, or
ducts, run the whole length of the zone. The hot gases travel
through the tubes or ducts which then radiate heat to the products
from above and below. At the end of the zone, the hot gases are
collected in a return duct through which they travel back to the
circulating fan and from there to the burner tube to be
re-circulated. It is essentially a closed, circulating system with
a single burner, circulating fan and radiant tubes to heat the
products from above and below.
Indirect fired radiant heating systemA flue with natural
convection is used to balance the pressure in the system resulting
from the ingress of combustion air at the burner. The continuous
re-circulation of the hot gases ensures a good efficiency. Fresh
air is only drawn into the system at the burner for combustion and
this is balanced by the natural extraction through the burner
flue.Since the products of combustion do not enter the baking
chamber, the burner may use oil or gas. This system is commonly
used where oil is the most economic fuel, for example in India.
Baker Perkins cyclotherm oven with turbulence tubesThis baking
system bakes by radiation with a high heat mass providing stable
baking conditions. It is versatile, capable of baking all types of
biscuit, cookies and some crackers. High rate crackers require a
first zone of direct heating. The system is favoured by many bakers
for producing a high quality of biscuit structure, texture and
colour. It is an ideal system for achieving colour contrasts on
rotary moulded and cracker products.
Closed circulation system
Summary
1.Indirect radiant ovens provide a very stable, radiant heat,
preferred by many bakers.2.The closed circulation system retains
and re-circulates the hot gases from the burner and contributes to
good fuel efficiency3.Suitable for all types of products, except
for the first zone(s) of ovens for baking high rate crackers (for
example soda crackers) which required direct heating4.Suitable for
all types of fuel including diesel oil2. Conduction heat transferAs
noted above, steel baking bands and heavy mesh bands conduct heat
rapidly into the base of the dough pieces. These types of band can
be used in any of the oven designs, direct gas fired, indirect
radiant ovens and convection ovens.Steel bands are made of carbon
steel, usually 1.2mm thick. They are principally used for the
baking of cookies with high sugar and fat contents, which flow on
the oven band in the first part of the oven. Traditionally steel
bands are also used for the baking of Marie.
Cookies deposited directly onto a steel band
Ashworth CB5 bandHeavy mesh baking bands, mainly Ashworth type
CB5, are woven with a tight herring bone pattern providing a solid,
thick, heavy mesh. These bands are pre-heated to 120 oC 150 oC and
they conduct heat immediately into the base of the dough piece as
soon as it is deposited on the band. This is a major baking method,
being used throughout the industry for the baking of soda crackers
and saltines. These bands are also versatile and can be used for a
wide range of crackers, hard sweet and rotary moulded products.3.
Convection bakingConvection ovens are constructed in zones, each
zone having a single burner and circulation fan.The fan blows the
air around the burner tube, where it is heated and then through
ducts along the length of the zone. These ducts, located above and
below the baking band, have slots or nozzles through which jets of
hot air are blown onto the products and the oven band. Hot air from
the baking chamber is drawn back to the fan to be re-circulated
through the system. Each zone has an extraction fan and flue to
remove moisture from the baking chamber and this system will also
extract the hot air from the heating system.Direct convection
ovensThe hot gases from the burner are combined with the
re-circulated air from the baking chamber and blown through the
ducts in the baking chamber and directly onto the products. As the
products of combustion are blown directly onto the products, diesel
oil fuels are unsuitable for direct convection baking.
Meincke direct convection systemIndirect convection ovens
The burner fires into a burner tube connected to a multi-pass
heat exchanger. The products of combustion are circulated within
the heat exchanger and do not enter the baking chamber. Air is
drawn from the baking chamber through the circulating fan and then
passed through the heat exchanger, where it picks up heat, before
being blow through the ducts in the baking chamber and onto the
products. This system can utilise gas or diesel oil fuels.
Meincke Indirect Convection oven with heat exchangerSummary
1.Convection ovens bake by simply blowing hot air onto the
product surface, so this is not a heat transfer system which
optimises the development of biscuit volume and
structure.2.Unsuitable for the first zone(s) of the oven as the
convective air dries and skins the surface of the dough pieces
preventing good lift and expansion3.Effective system for drying
dough pieces to low moisture contents. But this rapid drying occurs
at the product surface and may contribute to an excessive moisture
gradient in the biscuit and consequent checking or cracking of the
packaged biscuits.4.Provides even, bland colouring of the biscuits
without contrasts5.Direct convection ovens are relatively low cost
ovens to construct and control.Re-circ ovensThese ovens were
developed in USA as a versatile cookie and biscuit oven. The system
is basically a direct convection oven, but the volume of hot gases
blown directly onto the products can be adjusted. When this is
reduced, the hot gases circulate through the ducts at the top and
bottom of the oven and return to the burner tube, with less hot air
being blown onto the products. The system can therefore balance the
heat transfer by convective air or by radiation from the
ducts.Re-circ ovens may be used for products requiring lower heat
inputs. For the first zones, the oven is operated in a mainly
radiant mode to avoid skinning the dough piece. However the heat
transfer in this mode is low and the biscuit structure is
relatively slow to form. In the convective mode the heat transfer
is increased and the system is a direct convection oven.
Baker Perkins / Werner Lehara Re-Circ OvenSummary1.Suitable for
general baking of products requiring relatively low heat
inputs.2.Unsuitable for diesel oil fuelsHybrid ovensIt has become
common practice to combine different oven types into a hybrid or
combination oven. This allows the baker to use different heat
transfer modes at different stages of the baking process.Direct Gas
Fired / Indirect Radiant OvensProducts such as crackers and hard
sweet biscuits require high heat inputs in the first part of the
baking process to establish good structure and volume. This can
only be provided effectively by a direct heating system and a
direct gas fired oven section is normally specified. This system
also minimises the drying and skinning of the surface of the dough
pieces, which would prevent the lift and expansion of the dough
pieces when it is required.The length of this direct fired section
is usually one third of the total length of the oven and the power
input of the direct fired section is one half of the total power
input of the oven.The indirect radiant oven section will contribute
to the optimum development of texture and colour of a wide range of
crackers and biscuits.
Baker Pacific Combination oven: Direct Gas Fired / Indirect
RadiantDirect Gas Fired / Convection OvensThis specification again
uses the benefits of the direct gas fired oven in the first third
of the oven. The convection section will effectively remove
moisture from the dough pieces and achieve a low and even moisture
content for the final product. Colour will be even and bland,
without contrasts, which is suitable for a range of hard sweet
biscuits.
Dingson (DSM) hybrid Direct Gas Fired / Convection ovenMulti
system ovensOvens utilising three systems, for example direct gas
fired/indirect radiant/convection have been installed. However the
theoretical benefits are outweighed by the complexity of set up and
control and the relatively shorter sections of each
system.Dielectric bakingDielectric dryers have been manufactured by
theStrayfield Companyin UK and used in the biscuit baking industry
for almost 50 years. These are very effective in reducing moisture
content and in achieving very even moisture contents across the
width of the oven band. They are particularly effective on hard
sweet biscuits which are prone to checking (cracking after baking
due to stresses created by moisture gradient between the outer
surface and the centre of the biscuit).
Strayfield dielectric ovenThe Strayfield units are positioned
immediately after the conventional oven and consist of a separate
baking chamber, conveyor with synthetic band, and drying system.
The drying system comprises a transformer, rectifier, oscillator,
circuit with variable inductance and electrodes above and below the
product band.The dielectric system subjects the products to direct
heating from a radio frequency energy source. As the products pass
between the electrodes they are subject to a reversal of the
polarity which causes the molecules of water to rub together and
heat up. The heating rate increases as the frequency of the
reversal of the charge increases. High frequencies are used,
typically 27.12MHz.While the technology is effective, it is also
relatively costly in terms of both capital cost and operating cost.
It is therefore used mainly on high output lines where elimination
of checking is a major and quantifiable benefit.Microwave
bakingMicrowave units have been successfully tested in conventional
ovens. The addition of microwave units significantly speeds texture
and volume development and aids moisture loss from the centre of
the product. However the cost of generating the microwaves and the
safety issues involved have prevented the wider application of
microwave baking in the industry.
